1. Field of the Invention
This invention relates to polymer dispersed liquid crystal films and devices, and to methods of forming the same by controlling the conditions of photopolymerization to control the properties of the film.
2. Description of the Related Art
A new type of material referred to as a polymer dispersed liquid crystal (PDLC) film has been developed recently which has applications to various electro-optical switching devices, such as display modules for automobile dashboards, low voltage displays for optical shutters, and optical diffusers. The material is described in U.S. Pat. No. 4,688,900, issued Aug. 25, 1987 to Joseph W. Doane et al., and assigned to Kent State University. The PDLC film consists of droplets or bubbles of liquid crystal dispersed in a clear or light transmitting, flexible plastic sheet or film. The PDLC film is capable of being thermally, electrically, magnetically and electromagnetically addressed to cause the material to be reversibly switched between a light scattering mode and a light transmissive mode. The material is optically responsive to strain, so that under tension it acts as a polarizer that transmits one component of plane polarized light while scattering the other component. Curing the material in the presence of an electric or magnetic field also causes it to act as an electrically addressable polarizer.
The PDLC may be prepared by dissolving liquid crystal in an uncured monomer resin and then curing or polymerizing the resin so that bubbles or droplets of liquid crystal spontaneously form and are uniformly dispersed throughout the matrix. The exact "curing" method is not described, but it is known that PDLCs may be cured by a photopolymerization process of exposure to ultraviolet light. The patent also suggests that epoxy resins which are curable by UV radiation are useful. The resulting liquid crystal droplets are described in the patent as being of uniform size and spacing, and having a diameter ranging upward from about 0.2 microns depending primarily upon the curing procedure and the specific materials used.
The PDLC is described as exhibiting a reversible, high contrast optical response from an opaque scattering mode to a clear light transmission mode when either the temperature of the material is changed, or an electric field is applied. The material is optically responsive to strain, whereby under tension it acts to polarize incident light.
While the described PDLC represents a significant advance in film displays, further improvements would still be desirable. For example, the general approach described for modifying the properties of the film is to alter the materials used; a way to control the film's operating characteristics without having to tailor each different operating characteristic to a different set of materials would be highly desirable. Also, greater flexibility in the permissible uses of the film would be desirable. Existing PDLCs also typically require an operating voltage in the range of about 60-100 volts, and it would be advantageous to be able to significantly reduce this voltage level.